1. Field of the Invention
The present invention relates to a single-headed-piston type refrigerant compressor and, more specifically, to a single-headed-piston type refrigerant compressor provided with a plurality of single-headed pistons each being engaged by a pair of shoes with a cam plate mounted around a drive shaft to be moved linearly in a corresponding cylinder bore to compress a gas and an improved piston-bore fitting construction capable of both preventing the single-headed piston from rotating about its own axis and increasing a length of smooth slide-engagement between the piston and the corresponding cylinder bore.
2. Description of the Related Art
A conventional single-headed-piston type refrigerant compressor is provided with a plurality of single-headed pistons capable of being moved linearly in a plurality of parallel cylinder bores arranged on one side of the compressor body, a drive shaft, and a cam plate mounted around the drive shaft for rotation together with the drive shaft. Each of the single-headed pistons is engaged with the cam plate via a pair of shoes.
The single-headed-piston type refrigerant compressor includes a cylinder block in which the plurality of parallel cylinder bores are formed, a front housing covering a front end of the cylinder block and defining a crank chamber therein, and a rear housing defining a suction chamber and a discharge chamber and covering the rear end of the cylinder block. The drive shaft is rotatably supported by the cylinder block and the front housing.
When the cam plate of the single-headed-piston type refrigerant compressor is rotated to reciprocate the single-headed pistons, the cam plate exerts a force through the shoes to each single-headed piston, which has an essential axial component force effective in reciprocating the piston in the cylinder bore, and a rotative component force tending to turn the piston on its central axis. If the piston is turned about its central axis through a large angle by the rotative component force, a shoe-holding-portion of the piston comes, at a part thereof, into contact with a peripheral part of an outer circumference of the cam plate and, accordingly, the reciprocation motion of the piston is adversely affected.
Japanese Unexamined Patent Publication No. 8-109874 discloses a means for preventing the piston from rotating about its own central axis in which a partial circular projecting portion for preventing the piston from rotating is formed in a back face of the shoe-holding-portion of the piston to radially project toward an outer circumference of the piston. The outer surface of the partial circular projecting portion of the shoe-holding-portion of the piston is held in contact with an inner wall face of the housing defining the crank chamber, so that the rotation of the piston about its own central axis may be prevented.
Nevertheless, in the known single-headed-piston type refrigerant compressor, the cam plate exerts a side force tending to force the piston head portion in a radial direction of the compressor through the shoes during compressing and discharging strokes of the piston in addition to the axial component force and the rotative component force. The side force provides adverse affects on the smooth reciprocation of the piston and even produces a moment of force tilting the piston and causing abnormal abrasion of both the cylindrical outer circumference of the piston and the inner wall of the cylinder bore.
The moment of force tending to tilt the piston may be reduced by increasing the length of piston and that of the cylinder bore in order to increase a length of contact between the cylindrical outer circumference of the piston and the cylindrical inner wall of the cylinder bore. However, if a total length of the piston is increased to obtain a sufficient length of contact of the piston with the cylinder bore, the axial length of the cylinder bore must be correspondingly increased in a direction toward the rear housing for the purpose of securing a predetermined stroke volume of the piston. Consequently, an overall axial length of the compressor (i.e., the overall size of the compressor) must be increased.
On the other hand, if the length of a contact portion of the piston is increased by extending the cylinder bore in a direction toward the crank chamber opposing to the direction toward the rear housing, an amount of extension of the cylinder bore must be restricted by a relation between the afore-mentioned projecting portion of the piston and a cylinder-bore-forming portion of the cylinder block. Namely, it must be taken into consideration the fact that when the piston is moved to its top dead center within the cylinder bore, occurrence of mechanical interference between the projecting portion of the piston and the cylinder-bore-forming portion of the cylinder block must be avoided.